1. Field of the Invention
This invention relates to driver circuits for laser diodes, and more particularly to high-speed, power conditioning driver circuits for laser diodes and laser diode arrays.
2. Background
Laser diodes are continually finding new applications in the commercial, military, medical and other sectors. Laser diodes span the optical spectrum from the near infra-red (IR) through the visible wavelengths, which allows them to be used in a variety of applications, including, inter alia, optical communications, laser pointing and tracking, machining and welding, and pumping of a variety of optically-pumped lasers. Current technology trends all point toward expanded use of laser diodes, especially as efficiency and reliability are improved, and size and operating costs of laser diodes are reduced.
FIG. 1 is a schematic view of a conventional high power laser diode assembly 100 including an array of laser diodes 110. Array 110 includes laser diodes 102 arranged in parallel (rack) and series (stack). A xe2x80x9crack and stackxe2x80x9d approach enables the formation of arrays capable of generating high optical power densities (e.g., greater than 1 kW/cm2). Such arrays may require relatively high voltages (typically up to a few kilovolts) and high drive currents (typically up to a few kiloamperes) to operate. Array 110 is mounted on a micro-channel cooling plate 120 to dissipate heat generated by array 110. A one-dimensional or two-dimensional array of laser diodes is referred to herein as a laser diode array (LDA).
While laser diodes have been finding new applications, the breadth of these new applications has been limited by the cost of manufacture, test, and replacement of laser diodes and laser diode arrays.
Common sources of laser diode failure arise from excessive drive currents being provided to a laser diode in an attempt to achieve high laser efficiency (where efficiency is defined as optical power output as a ratio of electrical power input). Exemplary modes of laser diode failure resulting directly or indirectly from excessive drive current include (1) dislocation of and precipitation of host atoms from the laser diode semiconductor crystal, (2) oxidation of the laser diode mirror facets, and (3) metal diffusion of the laser diode electrode and wire bonds.
Controlling the drive current of laser diodes and laser diode arrays (LDAs) to avoid excessive current is complicated by the fact that laser diode junctions are highly nonlinear, dynamic electrical loads, and output optical power can change dramatically with only a small change in input current. One example mechanism of laser diode failure resulting in the modes of failure described above is voltage breakdown of a laser diode""s pn junction (also referred to herein as junction breakdown). Junction breakdown C. occurs when the drive current reaches a critical threshold, which causes strong optical absorption at a crystal defect. This in turn results in localized heating of the crystal, which causes its effective bandgap separation to shrink (and the voltage across a laser diode to decrease), giving rise to further optical absorption and increased drive current. This positive feedback process results in rapid thermal runaway, and breakdown of the pn junction.
Such voltage breakdown is illustrated graphically in FIG. 2, which shows a graphical representation of current versus time beginning with normal diode operation 210, followed by the onset of junction heating 220, during which time current increases and positive feedback begins. Ultimately catastrophic failure 230 occurs if current is not curtailed. Operation in a catastrophic failure regime can result in acute failure of a laser diode. A laser in which current has increased beyond that of normal diode operation is said to be in a xe2x80x9cfault state.xe2x80x9d
FIG. 3 is a schematic of a conventional power driver circuit 300 having an electrical power source 320 and a semiconductor switch 360 in series with an LDA 310. The pulsing of semiconductor switch 360 is controlled by a switch trigger circuit 365. Semiconductor switches used in conventional driver circuits have included power-field effect transistors (FETs) and integrated gate bipolar transistors (IGBTs).
One drawback of conventional power driver circuits, such as circuit 300 is that the laser diodes (or LDA) powered by the circuits may be exposed to excessive current or current densities in the laser diodes. For example, while switch 360 may limit the duration of excessive current to LDA 310 to prevent catastrophic failure, LDA310 may still be exposed to excessive current in the form of short peaks in current (i.e., transients), which occur over a period of time that is relatively short compared to the duration of pulses from switch 360 or the total current through the diode might constrict within the diode medium and produce local regions of excess current density.
Excessive current or current density may be generated by power source 320, or may be the result of changes in the operating conditions of an LDA such as constriction of the current in the laser diode medium, exposure to electromagnetic fields from other rill electric devices, electrical breakdown due to ionizing radiation from solar flares, cosmic rays or other sources of electric or magnetic interferences. Additionally, the current-voltage characteristics of an LDA itself may change over the operational lifetime of the LDA.
FIG. 4 is a graphical illustration of an exemplary current waveform 400 of a LDA driven by a conventional drive circuit. In FIG. 4, a semiconductor switch (e.g., switch 360 in FIG. 3) of the LDA driver circuit is turned on at time 410, and turned off 20 microseconds later at time 420. In exemplary waveform 400, during the 10 microsecond period 430 that follows time 420, the LDA is exposed to current transients 435. Even if junction breakdown does not occur, cumulative effect of exposure to such current transients may limit the lifetime of an LDA and cause premature failure.
To reduce the effect of transients and thereby increase the lifetime of LDAs, conventional driver circuits have been operated at reduced average currents and powers; however, reducing the current has resulted in a reduction of the optical output power available from a given LDA assembly, and has limited the applications for which a given LDA may be used.
Accordingly, there is a need for laser diodes and laser diode arrays that operate efficiently and provide adequate optical outputs over long lifetimes to reduce the costs per unit of lifetime. To that end, aspects of the present invention are directed to a driver circuit capable of providing improved transient protection to a laser diode source. Such driver circuits are capable of terminating excessive current or current density quickly in order to reduce premature laser diode failure. An additional advantage of such driver circuits is that they allow an associated laser diode source to be driven at a higher average driver current.
A first aspect of the invention is a laser diode driver circuit to generate a drive current, comprising a laser diode source to receive an amount of the drive current, an indicator device configured to receive an input signal corresponding to the amount of the drive current, and to generate an indicator signal indicative of the amount of the drive current received by the laser diode source, and a transient snubber device coupled to the indicator device to receive the indicator signal, that in response to the indicator signal is controlled to have a first impedance state during which a first amount of the drive current is provided to drive the laser diode source, and to have a second impedance state during which a second amount of the drive current is provided to drive the laser diode source, the second amount being less than the first amount.
In some embodiments of the first aspect, the second amount is substantially zero. The transient snubber device may be in parallel with the laser diode source or the transient snubber device may be in series with the laser diode source. Optionally, the transient snubber device may comprise a MOSFET transistor or a bipolar transistor.
In some embodiments, the laser driver circuit may further comprise a switch in series with the laser diode source to pulse the current provided to the laser diode source. The switch may be a GCT device.
The indicator device may be configured to receive the input signal that represents the amount of the drive current or the indicator device may be configured to receive the input signal that represents a voltage across the laser diode source. Alternatively, the indicator device may be configured to receive the input signal that represents a ratio of a voltage across the laser diode source to the drive current or the indicator device may provide the indicator signal that corresponds to a current density through the laser diode source.
In some embodiments the first aspect of the invention further comprises a prime power source coupled to the laser diode source to provide the electrical power to drive the laser diode source. The first aspect of the invention may further comprise a charging circuit coupled to the prime power source for receiving an output from the prime power source and for delivering the amount of drive current to the laser diode source. Optionally, the charging circuit may comprise a capacitive device to store a charge to deliver the amount of drive current or may comprise an inductive device to store an energy to deliver the amount of drive current.
A second aspect of the invention is a laser diode driver circuit to generate a drive current, comprising a laser diode source, a current source coupled to the laser diode source, to produce a source current, and an inductive device coupled to the current source, the inductive device storing energy corresponding to the source current, and the inductive device being coupled to the laser diode source to discharge the energy and thereby provide the drive current.
In some embodiments of the second aspect, the inductive device may comprises a first inductor in series with laser diode source, and a second inductor in parallel with the laser diodes source. In other embodiments, the driver circuit may further comprise a switch having a first state and a second state, which is coupled to the inductive device such that during the first state the inductive device stores the energy corresponding to the source current, and during a second state the inductive device discharges to provide the drive current. The switch may be a GCT.
A third aspect of the invention is a laser diode driver circuit to generate a drive current, comprising a laser diode source to receive an amount of the drive current, a means for generating an indicator signal indicative of the amount of the drive current; and a means for suppressing at least a portion of the drive current in response to the indicator signal, coupled to the means for generating an indicator signal, that in response to the indicator signal is controlled to have a first impedance state during which a first amount of the drive current is provided to drive the laser diode source, and to have a second impedance state during which a second amount of the drive current is provided to drive the laser diode source, the second amount being less than the first amount. The second amount is substantially zero.
In some embodiments, the means for suppressing is in parallel with the laser diode source. Alternatively, the means for suppressing may be in series with the laser diode source. Optionally, the means for suppressing may comprise a MOSFET transistor or a bipolar transistor.
Some embodiments of the third aspect of the invention further comprise a switching means in series with the laser diode source to pulse the current provided to the laser diode source. The switching means comprises a GCT device.
The means for generating the indicator signal may comprise a means for receiving an input which is representative of the amount of the drive current. Alternatively, the means for generating the indicator signal may comprise a means for receiving an input which is representative of the amount of a voltage across the laser diode source. The means for generating an indicator signal may be configured to receive the input signal that represents a ratio of a voltage across the laser diode source to the drive current.
The driver circuit may further comprise a prime power source coupled to the laser diode driver circuit to provide the electrical power to drive the laser diode source. The driver circuit may further comprise a charging circuit for storing the electrical power and for delivering the amount of drive current to the laser diode source. The charging circuit may comprise a capacitive device to store a charge to deliver the amount of drive current or may comprise an inductive device to store the electrical power to deliver the amount of current.
A fourth aspect of the invention is a method of driving a laser diode source with an amount of current, comprising generating a drive current, providing an amount of the drive current to the laser diode source, generating an indicator signal indicative of the amount of the drive current, and reducing the amount of the drive current provided to the laser diode source in response to the indicator signal indicating that the drive current exceeds a threshold.
In some embodiments, the act of reducing comprises shunting at least a portion of the current away from the laser diode source. Alternatively, the act of reducing comprises blocking at least a portion of the current from reaching the laser diode source. The method of driving may further comprise pulsing the drive current.
The act of generating the indicator signal may comprise receiving an input which is representative of the amount of the drive current or may comprise receiving an input which is representative of a voltage across the laser diode source.